(1) Field of the Invention
The invention relates to the field of the manufacturing of semiconductor integrated circuits, and more specifically to the aspect of dielectric insulation between capacitors fabricated for DRAM cells.
(2) Description of Prior Art
The process technology in the manufacturing of Dynamic Random Access Memory (DRAM) has, during the last several decades, migrated from 0.8 um 4M memories to 0.25 um 256M memories with a continuing decrease in memory cell size and concurrent increase in memory capacity. This scaling down in memory cell capacity puts increased emphasis on the dielectric isolation between the DRAM cells since this dielectric isolation impacts data retention capability.
One of the process technologies that have been used in the manufacturing of DRAM devices is the process of Selective Epitaxial Growth (SEG) of silicon. This process allows the deposition of a silicon epitaxial layer on a bare silicon surface without the simultaneous growth of amorphous silicon thin film on the silicon dioxide or silicon nitride interface. The SEG process has been used to develop an epitaxy-over-trench (EOT) process for DRAM technology. This approach allows the transfer transistor to be fabricated directly over the storage capacitor, resulting in a high density DRAM.
DRAM memory is so named because its cells can retain information only for a limited period of time before they must be read and refreshed at periodic intervals. A DRAM cell consists of one transistor and one storage capacitor. For bit densities of up to one megabit, planar-type storage capacitors are used. However, as storage densities increase, the amount of charges needed for a sufficient noise margin remains fixed. Therefore, in order to increase the specific capacitance, two different routes have been taken. The first is to store charges vertically in a trench. The second solution, which allows the cell to shrink in size without losing storage capacity, is to stack the capacitor on top of the access transistor. It is apparent from this that, as the memory density increases, the capacitor structure becomes more intricate and growth in the vertical direction. The present invention addresses the solution of storing charges vertically in a trench.
DRAM storage cell capacity can be increased by making the capacitor dielectric thinner, by using a dielectric with a larger dielectric constant or by increasing the area of the capacitor. The first two options are not currently available since capacitor dielectrics thinner than those now being used in DRAM cells will suffer leakage due to an electron tunneling effect. Dielectrics with significantly larger dielectric constants than that of SiO.sub.2 have not yet been accepted for DRAM application.
It must also be noted that since the 256-kbit DRAM generation bilayer films (consisting of both silicon nitride and SiO.sub.2) have been used as the capacitor dielectric to increase cell capacitance. The higher dielectric constant of Si.sub.3 Ni.sub.4 (twice as large as that of SiO.sub.2) was responsible for this increase.
The approach of storing charges vertically in a trench results in stacking the storage capacitor on top of the access transistor. The lower electrode of the stacked capacitor (STC) is in contact with the drain of the access transistor whereby the bit line runs over the top of the stacked capacitor. For STC cells to be made feasible for larger capacity DRAM's, an insulator with a larger dielectric constant than that of SiO.sub.2 must be used.
U.S. Pat. No. 5,362,666 (Dennision) and U.S. Pat. No. 5,405,796 (Jones, Jr.) show crown capacitor with conventional insulation between capacitors.
U.S. Pat. No. 5,798,289 (Ajika et al.) discloses several embodiments of capacitors.